This invention relates to fiber Bragg grating dispersion compensation, and particularly to a thermally tunable fiber Bragg grating dispersion compensator.
One of the key issues in modern high-speed optical networks is the necessity to compensate for the optical pulse broadening caused by optical fiber chromatic dispersion. With the advance of new generations of fast networks (40 Gb/sec and higher), the ability to precisely compensate for the dispersion becomes critical for the network operation thus necessitating dispersion compensation components with variable dispersion capabilities.
Efforts to compensate for chromatic dispersion have involved thus far the use of etalon-based systems, dispersion compensating fibres, dispersion compensating gratings, e.g. fiber Bragg gratings (FBG), or a combination of both. A device described in a paper xe2x80x9cImplementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradientxe2x80x9d, J. Lauzon et al, Optics Letters, Vol. 19, No. 23, pp. 2027-2029, December 1994, has a heat distributor and thermoelectric coolers to control the end temperatures of the distributor.
Various dispersion compensating systems are also described in patent literature, e.g. U.S. Pat. No. 5,671,307 issued to Lauzon et al., U.S. Pat. No. 6,148,127 issued Nov. 14, 2000 to Adams et al, U.S. Pat. No. 5,694,501 issued Dec. 2, 1997 to Alavie et al. (now assigned to the present assignee), and U.S. Pat. No. 6,307,988 issued Oct. 23, 2001 to Eggleton et al.
It is desirable to provide a tunable dispersion compensator (DC), preferably over a broad dispersion range. The key element of a popular type of a DC is a linearly chirped fiber Bragg grating (FBG), a diffractive grating with a linearly varying pitch (refractive index perturbation) written inside an optical fibre. Optical pulse broadening comes from the fact that the pulse""s frequency components travel with different velocities, so that the longer wavelength components lag the shorter ones. In a chirped FBG, the location of resonant Bragg condition (reflection point) will be wavelength dependent. This causes a time-of-flight difference between longer and shorter wavelength equivalent to a chromatic dispersion added to the pulse.
Since silica used for fibre manufacturing has a temperature dependent refractive index and the fibre itself has certain thermal expansion coefficient, the grating local resonant wavelength becomes temperature dependent and varies as xcex94xcex=STxcex94T , where STxe2x89xa110 pm/K is the grating thermo-optical sensitivity.
In a linearly chirped grating, i.e. grating with the reflection position varying linearly with wavelength, group delay will be a linear function of wavelength and, after differentiation, yields uniform dispersion across the grating wavelength range (bandwidth). Any deviations of group delay from the linear profile called group delay ripple (GDR) distort the shape of an optical pulse and thus they are highly undesirable.
If one creates a uniform temperature gradient along a linearly chirped FBG, the grating chirp changes but remains linear thus giving rise to a different dispersion value. Based on this fact one can design a dispersion compensator with a thermally tuneable dispersion. Unfortunately, as is commonly known, an elongated object heated at two ends, due to thermal losses, will exhibit a non-linear temperature profile, the temperature deviation from linearity being greatest in the middle. In the case of a distributor housing a chirped Bragg grating, such thermal losses (dubbed here xe2x80x9ctemperature saggingxe2x80x9d) amount to an undesirable group delay ripple (GDR).
It is desirable to provide a DC capable of maintaining a uniform temperature gradient on the FBG, typically a chirped FBG.
It is further desirable to provide a DC with means for varying the temperature gradient at a relatively high rate, preferably maintaining the temperature gradient linearity.
In accordance with one aspect of the invention, there is provided a dispersion compensator comprising: a length of a waveguide including a grating region having two opposite ends, a heat distributing body extending along the grating region and adjacent to the grating region, and a heating strip mounted to the body and extending along the grating region for controlled heating of the entire grating region.
The compensator may have a temperature sensor disposed intermediate the ends of the grating region, preferably at the center of the grating region, for generating a signal indicative of the temperature of the respective region of the distributor. Preferably, the grating region is a chirped Bragg grating, e.g. a linearly chirped Bragg grating to afford an efficient compensation of chromatic dispersion.
The compensator may further include a longitudinally variable heating means adjacent to and extending along the length of the distributing body and the grating region for effecting a longitudinally varying heating of the grating region, the heating means having a monotonic heating-intensity variance along the length of the grating region.
In one embodiment, the compensator has two terminal heating/cooling means adjacent the ends of the grating region for heating at least the end parts of the grating region.
As indicated above, it is desired to create a linear temperature profile along the Bragg grating. To this end, according to the invention, the grating is disposed in a close proximity of a heat distributor, preferably inside a heat distributor. Tests have shown that the provision of the distributor, made of a material of high thermal conductivity and preferably but not necessarily with a thermal expansion coefficient (CTE) identical or close to CTE of the material of the grating (glass), is beneficial in maintaining a linear variance of temperature along the grating.
Dispersion dependence on temperature difference at the ends of the grating can be expressed as   D  =            (                        1                      D            0                          +                                            cS              T                                      2              ⁢              n                                ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              T                        L                              )              -      1      
where D0 is the dispersion of the grating without temperature gradient (xe2x80x9cnominalxe2x80x9d dispersion), L is the grating length, n is refractive index of the optical fibre in which the grating is imprinted, ST is as explained above and c is the speed of light. Based on the desired dispersion tuning range, one can calculate the required temperature range. In order to maintain the device""s bandwidth centred at a particular wavelength, the grating center temperature should remain constant. A sensor, e.g. a thermistor placed at the centre of the distributor provides the necessary feedback for the uniform heater control loop. For a linear temperature profile, the temperature at the middle of the distributor (and thus at the middle of the grating region) should be maintained at (T1+T2)/2 where T1 and T2 are the temperatures at the respective ends of the distributor. A central control unit may be provided to respond to the signal generated by the sensor.